1. Field of the Invention
The present invention relates to expandable polystyrenic resin particles (beads) and a production process thereof, pre-expanded particles and a molded foam product. More particularly, the expandable polystyrenic resin particles of the present invention employ a core-shell structure (see FIG. 1) having a surface layer (shell), and together with maintaining high expandability over a long period of time due to favorable retention of blowing agent, allow the obtaining of a molded foam product having superior resistance to cracking even if the blending ratio of polystyrenic resin is low.
Priority is claimed on Japanese Patent Application No. 2006-291116, filed on Oct. 26, 2006, the content of which is incorporated herein by reference.
2. Description of the Related Art
Impregnating polystyrene resin particles with a volatile blowing agent such as propane, butane or pentane allows the obtaining of expandable polystyrene resin particles (EPS) that have the ability to foam. Expandable polystyrene resin particles have favorable retention of blowing agent, can be stored at room temperature or in a refrigerated state, can be heated to form pre-expanded particles at appropriate times, and can be molded into molded foam products by filling into a mold of a molding machine and heating. Since this molded foam product has superior heat insulating properties, cushioning properties and light weight, it is used in a wide range of applications including food packages such as fish containers, cushioning materials for home appliances, and heat insulating materials for construction materials. However, these polystyrene resin molded foam products have the problem of being susceptible to cracking due to impacts and the like, thereby limiting further expansion of their applications.
On the other hand, molded foam products composed of polyolefin resins such as polyethylene resin or polypropylene resin are known to be flexible and resistant to cracking (superior cracking resistance) in addition to having the characteristics of polystyrene resin molded foam products. However, since polyolefin resins have inferior blowing agent retention, they cannot be stored as expandable resin particles. Moreover, since foam molding conditions are required to be precisely controlled, they also have the problem of high production costs.
In order to solve the aforementioned problems, a molded foam product has been proposed that compounds a polystyrenic resin and polyolefin resin.
Patent Document 1 (Japanese Unexamined Patent Application, First Publication No. S54-119563) proposes secondary expandable foam particles in which the surface layer is composed of a foamed polyolefin resin while the core section is composed of foamed polystyrenic resin for the purpose of realizing the characteristics of both polystyrenic resin and polyolefin resin by mutually compensating for the shortcomings thereof.
In addition, Patent Document 1 describes that, according to these foam particles, moldability is superior since blowing agent is retained in the foamed polystyrenic resin of the core sections. Moreover, it is described that a molded foam product obtained from these foam resin particles has superior rigidity, flexibility and low-temperature properties.
However, as a result of the inventors of the present invention carrying out additional testing on the resin particles described in Patent Document 1, even if blowing agent was impregnated into the resin particles comprised of polystyrenic resin particles coated with polyolefin resin and immediately subjected to pre-expanding, only the internal polystyrenic resin underwent considerable foaming, while the polyolefin resin only exhibited slight foaming or no foaming at all. In addition, as a result thereof, separation occurred at the boundary between the surface polyolefin resin layer and the polystyrenic resin, thereby preventing the obtaining of the target molded foam product. In addition, due to prominent escape of blowing agent from the surface layer, it was difficult to store in the state of expandable resin particles. Thus, these foam particles have not been able to be used on an industrial scale.
In addition to the method described above, several expandable resin particles have been proposed in which polystyrenic resin is modified with polyolefin resin by impregnating a styrenic monomer into polyolefin resin particles and polymerizing an in aqueous medium.
For example, in Patent Document 2 (Japanese Examined Patent Application, Second Publication No. H7-45590), the inventors of the present invention proposed expandable styrene-modified polyethylene resin particles comprising impregnating a readily volatile blowing agent into styrene-modified polyethylene resin particles containing 0.01 to 5% by weight of water-containing silicon dioxide.
However, the properties of these modified expandable resin particles and molded foam products thereof are greatly affected by the ratio of the polystyrenic resin and polyethylene resin. Namely, although resistance to cracking of the molded foam products improves the higher the ratio of polystyrenic resin, since retention of blowing agent decreases rapidly, the period during which the expandable resin particles can be foam molded (to be referred to as bead life) is shortened, and it was difficult to realize both resistance to cracking and bead life.
In addition, Patent Document 3 (Japanese Unexamined Patent Application, First Publication No. 2006-70202) proposes styrene-modified polyethylene expandable resin particles containing 300 to 1000 parts by weight of a styrene resin component and a volatile blowing agent to 100 parts by weight of a polyethylene resin component containing an inorganic nucleating agent and having a melting point of 95 to 115° C., and in a state in which styrene resin particles of 0.8 μm or less are dispersed in a surface layer to at least 5 μm from the surface of the particles.
Although these modified expandable resin particles have improved bead life as a result of the styrene resin component being present in excess than the polyethylene resin component, the object of the present invention is unable to be achieved due to inadequate resistance to cracking.
In addition, the inventors of the present invention proposed foam particles of polystyrenic resin modified with polyolefin resin and a production process thereof in Patent Document 4 (International Publication No. WO 2005/021624).
The base resin particles of these foam particles were produced by adjusting the impregnation polymerization conditions of the styrenic monomer, namely adjusting the required stirring force to within a predetermined range, and adjusting the content of styrenic monomer in the polyolefin resin particles to a predetermined amount. Since polymerization progresses in the state in which the amount of styrenic monomer gradually decreases from the core sections towards the surface of the polyolefin resin particles, and further progresses while styrenic monomer is sequentially absorbed into polystyrenic resin formed in the polyolefin resin particles, the polyolefin resin particles become richer in polystyrenic resin the closer to the core sections accompanying formation of polystyrenic resin, while the surface is in a state in which polyolefin resin is present at a higher ratio.
In these foam particles, although polystyrenic resin is contained at a high ratio in the core sections thereof, at locations near the surface, polyolefin resin is contained at a high ratio while the ratio of polystyrenic resin gradually decreases moving towards the surface of the particles, thereby resulting in a state of being finely dispersed within the polyolefin resin, while the particle surface is in a state in which there is hardly any polystyrenic resin present.
Since a resulting molded foam product has a structure in which the entire surface thereof is covered with polyolefin resin containing a high ratio of pre-expanded particles near the surface thereof, the molded foam product is provided with superior resistance to cracking as well as chemical resistance.
However, since there is no change in the tendency for retention of blowing agent to decrease easily even if this technology is applied, it is difficult to maintain the bead life of the expandable resin particles for a long period of time, and it is still difficult to easily realize both resistance to cracking and bead life.
In addition, Patent Document 5 (Japanese Unexamined Patent Application, First Publication No. H2-284905) discloses a method that uses a water-soluble polymerization inhibitor in the production of carbon-containing styrene-modified expandable polyolefin resin particles by supplying styrenic monomer to an aqueous medium in which olefin resin particles containing electrically conductive carbon particles are dispersed in water, suspension polymerizing the styrenic monomer, and impregnating a volatile expansion agent into the particles.
The water-soluble polymerization inhibitor inhibits polymerization of vinyl monomers containing styrene near the surface of particles while dissolving in an aqueous medium and being absorbed by carbon-containing olefin resin particles. As a result, since the surfaces of the resulting styrene-modified expandable polyolefin resin particles have a lower amount of polystyrene than the particle centers, while conversely the surfaces having a higher content of electrically conductive carbon, styrene-modified olefin resin particles are obtained that have superior electrical conductivity.
As has been described above, a water-soluble polymerization inhibitor has the effect of inhibiting polymerization of styrenic monomer on the surface of polyolefin resin particles, and in that sense, makes it possible to improve the blackness and electrical conductivity of a molded foam product. However, since inhibition of styrenic monomer polymerization is limited to only the surface, effects for improving resistance to cracking are inadequate. Accordingly, since the ratio of polyolefin resin in modified resin particles cannot be lowered, retention of blowing agent is not enhanced and similarly the object of the present invention is unable to be achieved.    [Patent Document 1] Japanese Unexamined Patent Application, First Publication No. S54-119563    [Patent Document 2] Japanese Examined Patent Application, Second Publication No. H7-45590    [Patent Document 3] Japanese Unexamined Patent Application, First Publication No. 2006-70202    [Patent Document 4] International Publication No. WO    [Patent Document 5] Japanese Unexamined Patent Application, First Publication No. H2-284905
With the foregoing in view, an object of the present invention is to provide expandable polystyrenic resin particles, which together with having superior resistance to cracking for molded foam products, that was a problem for expandable polystyrenic resin particles of the prior art, maintains high expandability over a long period of time, a production process thereof, and a molded foam product.